When cables or the like are sheathed with a plastic material, extruders are used, which plastify the plastic material and inject it round about a core (conductor). The plastic material is made soft by pressure and heat in the extruder, so that it can be applied to the core without problems and without inclusions. For this purpose, it is desired to determine the temperature in the plastified material, in order to achieve an optimum viscosity of the melt for the extrusion process. Furthermore, burning or premature crosslinking in the extruder head must be avoided. Instead, the crosslinking process has to take place immediately after the extrusion. Thus, a reliable temperature measurement optimises the production process.
In such extrusion processes, it is known to measure the temperature of the material at different delivery speeds outside of the extruder head with the aid of a thermocouple. This measurement takes place at a delimited location, and of course it has a relatively long response time. The thermocouple measures only the temperature in the outer region of the melt. If however the thermocouple projects into the melt, the determination of the temperature is distorted by sliding friction effects. Besides to that, the thermocouple negatively affects the flow properties of the melt.
It is also known to measure the temperature of the melt with the aid of an infrared thermometer. However, by doing so, the surface temperature of the melt can be acquired only up to a depth of a few millimetres. When there are special fillers in the melt, the measurement depth is reduced even further.
However, a capture of the mass temperature of plastics melts that is as accurate as possible can help to increase the production output of an extrusion plant significantly, namely in that amongst others the idle time (for instance cleaning of the plant, repair etc.) of the plant is reduced by selecting an optimum temperature.
Thus, the present invention is based on the objective to indicate a method for the measurement of the temperature of at least one plastified plastic material at the exit of an extruder, which permits an accurate non-contact, non-invasive temperature measurement of a melt in a simpler way.